1. Field of the Invention
The present invention relates to a dielectric thin film element used in a memory device, a pyroelectric device, a piezoelectric device, an electrooptic device, etc. Further, the present invention relates to a method for manufacturing the dielectric thin film used in such a dielectric thin film element.
2. Description of the Related Art
Recently, non-volatile semiconductor memory elements are attracting much public attention. These non-volatile semiconductor memory elements are represented by a read only memory (ROM), a programmable ROM (PROM), an erasable PROM (EPROM), an electrically erasable PROM (EEPROM), etc. In particular, the EEPROM is promising since stored contents of the EEPROM can be electrically rewritten. In the EEPROM, a trap region or a floating gate within a gate insulating film of a metal insulator semiconductor field effect transistor (MIS-FET) is charged by charge injection from a silicon substrate. The surface conductivity of the substrate is modulated by electrostatic induction of injected charges.
A ferroelectric non-volatile memory utilizing spontaneous polarization of a ferroelectric substance can be used as a non-volatile memory using a method completely different from the above known modulating method of the EEPROM. This ferroelectric non-volatile memory can have one of two structures, either a structure composed of an MFS (metal ferroelectric semiconductor)-FET (field effect transistor) structure or a capacitor structure.
In the MFS-FET structure, a gate insulating film of the MIS-FET is formed as a ferroelectric thin film. The conductivity of a semiconductor surface is modulated by charges induced on the semiconductor surface such that spontaneous polarization of a ferroelectric material is compensated in accordance with the direction and the magnitude of the spontaneous polarization. Memory contents are read by utilizing this modulation.
In the capacitor structure, a ferroelectric thin film is supported between two electrodes. Memory contents are read by detecting the existence or nonexistence of an inverted electric current caused by inverting the spontaneous polarization of the ferroelectric material.
For example, the ferroelectric material used for such a ferroelectric non-volatile memory can be lead zirconate titanate (which is called PZT in the following description), lead titanate (PbTiO.sub.3), or barium titanate (BaTiO.sub.3), etc. At present, the PZT material is being vigorously researched as a material for a most promising non-volatile memory.
A dynamic random access memory (DRA) appeared in 1970 as a charge storing solid state memory having a simple structure and manufactured by a simple manufacturing method. This DRAM is widely used while the degree of integration of the DRAM is increasing. In particular, a so-called 1Tr-1C DRAM is formed by constructing one capacitor and one transistor in one memory cell. After 1972, this 1Tr-1C DRAM has been most widely used since this DRAM has a simple shape and a small size. In this DRAM, a dielectric thin film capacitor stores charges and a transistor composed of a semiconductor is used as a switch for separating capacitors from each other.
The chip area is gradually increased as the DRAM is integrated. However, the cell area is further reduced in comparison with the increase in chip area. It is necessary to set a cell capacity (as an electrostatic capacity of the capacitor) required for the DRAM to about 30 fF (3.times.10.sup.-14 F) in consideration of sensitivity of a sense amplifier, a bit line capacitance, etc. even when the DRAM is integrated. An effective area S for the capacitor must be increased, or the thickness d of a capacitor thin film must be reduced to maintain an electrostatic capacity C of the capacitor at a certain value. Otherwise, a dielectric constant .epsilon. of a capacitor material must be increased to maintain the electrostatic capacity C of the capacitor at a certain value. Therefore, the effective area S for the capacitor is increased and the thickness d of the capacitor thin film is reduced with respect to a stacked type capacitor cell, a trenched type capacitor cell, etc. as a method for increasing the integration degree of the DRAM until the present time. However, in such a stereoscopic structure, the number of manufacturing processes increases and the yield is reduced by an increase in step height. Further, techniques for forming a thin film have already reached a limit. Accordingly, it is necessary to deposit a thin film of a dielectric substance having characteristics of a high dielectric constant .epsilon. on silicon (Si).
Changes in residual polarization and dielectric constant with respect to a Zr/Ti ratio in PZT are described in "Properties of D.C. magnetron-sputtered lead zirconate titanate thin films" Thin Solid Films, 172 (1989), pp. 251-267. In this case, the Zr/Ti ratio ranges from 50/50 to 70/30 and the residual polarization is set to Pr.gtoreq.10 .mu.C/cm.sup.2.
However, dielectric characteristics of a dielectric thin film constituting the above general dielectric thin film element are not necessarily sufficient to obtain a practical dielectric thin film. In particular, it is necessary to reduce the leakage current generated between both electrodes supporting the dielectric thin film. Further, it is necessary to improve the electrical fatigue characteristics of the dielectric thin film. For example, a fourth element is added as a trial to a PZT material as a dielectric thin film material so as to improve the above electrical characteristics of the dielectric thin film. However, no dielectric thin film having high performance has been realized so far.